Electron Transport Chain

Electron Transport Chain
Transfer of hydrogen ions and electrons
from reduced NAD and FAD to
the final electron acceptor, molecular
oxygen, is accomplished in an elaborate
electron transport chain embedded
in the inner membrane of mitochondria
(Figure 4-14).
Each carrier molecule in the chain
(labeled I to IV in Figure 4-14) is
a large protein-based complex that accepts
and releases electrons at lower
energy levels than the carrier preceding
it in the chain. As electrons pass
from one carrier molecule to the next,
free energy is released. Some of this
energy drives the synthesis of ATP by
setting up a H+ gradient across the
mitochondrial membrane. At three
points along the electron transport system,
ATP production occurs by phosphorylation
of ADP. By this means,
oxidation of one NADH yields three
ATP molecules. Reduced FAD from the
Krebs cycle enters the electron transport
chain at a lower level than NADH
and so yields two ATP molecules. This
method of energy capture is called oxidative phosphorylation because
the formation of high-energy phosphate
is coupled to oxygen consumption,
and these reactions depend on
the demand for ATP by other metabolic
activities within the cell.

How is ATP actually generated
during oxidative phosphorylation? The
most widely accepted explanation is a
process called chemiosmotic coupling
(Figure 4-14). According to this model,
as electrons contributed by NADH and
FADH2 are carried down the electron
transport chain, they activate proton
pumping channels which pump protons
(hydrogen ions) outward and into
the space between the two mitochondrial
membranes. This causes the proton
concentration outside to rise, producing
a diffusion pressure that drives
the protons back into the mitochondrion
through special proton channels.
These channels are ATP-forming protein
complexes that use the inward
passage of protons to induce the formation
of ATP. Exactly how proton
movement is coupled to ATP synthesis
is not yet understood.

Figure 4-14 Oxidative phosphorylation. Most of the ATP in living organisms is produced in the electron transport chain. Electrons removed from fuel molecules in cellular
oxidations (glycolysis and the Krebs cycle) flow through the electron transport chain, the major components of which are four protein complexes (I, II, III, and
IV). Electron energy is tapped by the major complexes and used to push H+ outward across the inner mitochondrial membrane. The H+ gradient created
drives H+ inward through proton channels that couple H+ movement to ATP synthesis.